The use of gamma ray scattering measurements to measure the density of subsurface formations is well-known. Tools consist of a gamma-ray source and at least one detector. The earliest single detector tools were soon replaced by dual detector tools that allow compensations to be made for the possible intervening presence of mud or drilling fluid between the tool and the formation. Gamma rays from the source pass into the formation where they interact with electrons in the formation material. Some of the gamma rays eventually scatter back into the tool and are detected by instrumentation. The number of detected gamma rays depends on the number of electrons in the formation, and the number of electrons is proportional to the density of the formation. Consequently, the formation density can be determined from the number of detected gamma rays.
Modern tools also measure the energy of the detected gamma rays. Whereas high-energy gamma rays are primarily affected by simple scattering off of electrons (Compton scattering), low-energy gamma rays are affected both by Compton scattering and by photoelectric absorption. The industry has devised a quantity that exemplifies the amount of photoelectric absorption of a formation. That quantity, called the Photoelectric Factor and referred to as Pe, can be computed by combining the number of high-energy gamma rays and low-energy gamma rays that are detected by a particular sensor. Consequently, by measuring the energy of detected gamma rays, modern tools compute both the density and the Pe of the formation.
Gamma ray density measurement logging is typically performed by lowering an instrument down a borehole and recording gamma radiation at various depths. The instrumentation is surrounded by a pressure housing to protect it from the high pressures and the fluid encountered in the borehole. Density logs, which are the result of the series of gamma radiation measurements, are affected by the diameter of the borehole and the properties of the fluid filling the borehole.
Current gamma ray density measurement logging tools typically use “windows” in the pressure housing. Windows increase the transmission of gamma radiation into detectors. Each window, however, may require one or more o-rings to form a seal with the pressure housing, which is required to maintain the pressure barrier between the instrumentation and borehole. O-rings at high borehole pressures and temperatures are a liability. The o-rings must be changed often, especially after high-pressure, high-temperature jobs. Thus, it is best to have as few necessary o-rings as possible, and to ensure that they are easy to change. In existing systems, o-rings around windows are typically difficult to replace. In addition, the way that the o-rings must be implemented makes them a risk; it is possible for external pressures to extrude the o-rings out of position within grooves, resulting in a failure.
Existing systems provide a chemical source of gamma rays, which interact with the formation. Detectors then detect the resulting interaction and the interaction is analyzed to determine characteristics of the formation. Many existing systems, however, suffer from sensitivity to gamma radiation from the borehole.